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Thread: residential roofs

  1. #11
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    Default Re: residential roofs

    Quote Originally Posted by thomasf View Post
    The only problem is, if you look at your roof in section, one of the pitches is plating correctly, from the inside face of the stud wall (see section 2), but the other pitch is not. But, I just remembered the math and it is very simple actually. The overhang is the run (12") divided by the rise (x) times 12 = overhang in inches.

    so for a 12:12 pitch it's 12/12 = 1 x 12 = 12"

    7:12 would be 12/7 = 1.714285714285714 x 12 = 20.57142857142857"

    this is from outside face of stud, not face of finish material. So, i'm going to try using this overhang measurement & see if it works.
    That's what I'm saying, if you want aligned eaves, either one set of rafters will have to have larger bird's mouth cuts, or the other set of walls will have to be slightly taller or have additional blocking above the wall top plate. That's the only way, geometrically, to get aligned roof eaves.

    When I say aligned roof eaves, I'm referring to the UPPER point of the eave, i.e. where the gutter attaches.
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    Default Re: residential roofs

    Unfortunately, that doesn't help since I am not using trusses. Thanks anyway though.

    Using my math above I've gotten it pretty close, but still not correct. I hope this is something they keep working on cause this is a big obstacle to designers like myself who work on both commercial & residential projects from completely embracing Revit!

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    Default Re: residential roofs

    Quote Originally Posted by patricks View Post
    That's what I'm saying, if you want aligned eaves, either one set of rafters will have to have larger bird's mouth cuts, or the other set of walls will have to be slightly taller or have additional blocking above the wall top plate. That's the only way, geometrically, to get aligned roof eaves.

    When I say aligned roof eaves, I'm referring to the UPPER point of the eave, i.e. where the gutter attaches.
    I understand what you're saying, in order for it to be geometrically correct in Revit, you have to do one of the two things you mentioned. But in the field that's not how it works. The pivot point is where the roof rafter intersects the inside face of the stud, the bird's mouth cut is always 3 1/2" deep so the roof rafter will fully seat on a typical 2x4 wall, the horizontal cut at the rafter end is a fixed distance of 1'-0" from the top of plate, the vertical cut is a variable distance (based on the pitch) from the outside face of the stud. Using this method, the eaves will always align!

    I first looked at Revit in 2002 and the inability to draw a residential roof, the way it's built in the field, is what put me off from totally ditching AutoCAD, and 10 years later it hasn't gotten any better

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    Certified AUGI Addict patricks's Avatar
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    Default Re: residential roofs

    I sketched out the two conditions you described, put dimensions on them, and made the roof by manually specifying the overhangs for each side and then aligning eaves by adjusting plate height.

    I noticed when I created the roof first by picking all walls at 12/12 slope, changing 2 sides to 7/12, and then doing Align Eaves by adjusting the overhang (adjusting the 7/12 eave height to match the 12/12 eave height), the 7/12 overhangs went longer than they should, and the original 12/12 sides no longer aligned with the inside top plate. IMHO that shouldn't happen. Seems like a bug.

    Check out my revised file and take a look at the drafting view sketch.
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    Default Re: residential roofs

    Okay, I'm glad somebody understood what I was saying and was able to re-create the problem. I performed the exact same steps as you and got very similar results.

    I looked at your file and made some adjustments to the drafting view, as we use 2x6's for roof rafters. I also made a small adjustment to the wall type, again based on local building methods, the 1/2" sheathing on the exterior of the stud is included in the 5 1/2" brick ledge. I then attempted to re-create the roof, manually using the distances determined in the drafting view, and was unsuccessful. Even though each roof edge is set to define slope, the overhangs are correct and extend into core is checked, none of the rafters align with the inside top plate!

    Perhaps this is just something Revit isn't capable of doing, or as you said, a bug in the portion of the program that calculates overhangs, when you select the Align Eaves - Adjust Overhangs option.
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    Certified AUGI Addict patricks's Avatar
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    Default Re: residential roofs

    I got your file to work. I went to Section 1 and moved the bottom edge of the roof up to the inside edge of top plate. Then go to the drafting view and measure the vertical height of the rafter end (below that 7 xxx/256 dimension). Transfer that number to the Fascia Depth on your roof. That will give you the 12 inches from top plate to bottom corner of rafters, and the overhang dimensions will match the sketch.

    So you can make it work, but I think there's a bug when initially sketching the roof and adjusting eave overhangs and plate heights.
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    Default Re: residential roofs

    OK, I was able to get it to work to with that method and following the same the technique was able to get it to work after adding the roof decking & asphalt shingles. That is a lot of steps to get the roof to work though!

  8. #18
    Certified AUGI Addict patricks's Avatar
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    Default Re: residential roofs

    Yes it is. If I get a chance I'll be submitting a support request about it.

    However, I don't recall seeing houses with soffit depths that vary based on roof slope. I always see (and would rather have on our commercial jobs) soffits that are equal all around, equal height, etc. which would necessitate having the bearing point for the steeper slope higher than that of the shallower sloping roof, to get all eaves and soffits to align and be equal all around.

    Is the varied soffit/overhang depth a common thing in your area?
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  9. #19
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    Default Re: residential roofs

    Actually, my experience is the opposite of yours. I have never seen a builder change the bearing the point to create an equal depth soffit. Every homebuilder/framer, I've ever worked with, that frames with traditional stick framing as opposed to trusses, builds them as I've described, creating deeper overhangs to obtain alignment, and I've done houses from Virginia to Vegas.

    The varying pitches usually don't come into play until I get into move-up homes, most starter homes have equal pitches all the way around.

  10. #20
    Certified AUGI Addict patricks's Avatar
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    Default Re: residential roofs

    I've seen it plenty of times in upscale homes around here in the Southeast. There are home shows with 3500 - 5000+ SF homes where I'll go in the attic and see *very* steep rafters of up to 14/12 or even 16/12 sitting on a ~12" high framed knee wall above the wall top plate. Or in many cases I've seen a roof with 2 pitches, where the steeper pitch rafters are cut off right at or just past the outside face of framing, and then separate little rafter pieces are scabbed onto each main rafter at the lower roof pitch specifically to make the eaves and overhangs equal all around.

    For the life of me I can't think of a name for that condition and I can't seem to find a stinking image or diagram anywhere, but I see it a LOT around here.

    *edit* here's an image, though I think this one shows equal pitches on both sides, with equal "flare-out" pitches on both eaves. But I've seen this done with the flare out on a steeper side to match the eave to the adjacent shallower-sloped roof face. The portion of the hip just above the eave ends up as a 45° angle in plan view, but the rest of the hip above the interior of the house would not be at a 45 in plan view.

    Last edited by patricks; 2012-05-09 at 05:23 PM.
    Intern Architect, BIM Manager/Coordinator
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